From an excellent New Republic editorial on the false conflation between "angry" and "liberal":
To be fair, not all Democrats are as affectless and disciplined as [Hillary] Clinton. [Howard] Dean really is angry, as are demagogues like Michael Moore and Cindy Sheehan. But the problem with the Moores and the Sheehans is not that they're angry; it's that they're wrong. The Republicans have substituted a temperamental category for an ideological one. When they fail to demonstrate a philosophical failing, they try to demonstrate a human failing.Coming up next on Fox... angry Democrats! You'll be outraged!That's the true damage extracted by this Republican attack. They have defined anger down. A good, honest emotion has been trashed in pursuit of cheap political points...
There are also obvious commercial reasons for conservative commentators to expend so much energy denouncing anger. Rage, after all, is an essential part of the Fox News ethos, where Bill O'Reilly makes Howard Beale look like Dag Hammarskjöld. Pretty clearly, these conservative commentators will denounce Democrats as angry, because that will make their audiences angry, generating even larger angry audiences. And you know what that makes us.